Water heaters include an inner tank and an outer shell. The outer shell includes shell openings to allow fittings to pass through the shell to the inner tank. Further, in the case of an electrical water heater, there are additional shell openings allowing access to electrical components located on the inner tank.
According to conventional practise, water heaters are now insulated by foam insulation formed in the space between the tank and the outer shell. It is important that the foam not escape outwardly and mar the outer surface of the outer shell.
The most widely accepted method of foam insulating a water heater is through the use of a bag placed between the tank and the shell. The bag is open at the top of the water heater and the insulation forming material is sprayed down into the bag which prevents the foam from escaping outwardly of the shell.
The use of foam trapping bags is costly from a material standpoint. It is further costly from a labour standpoint in that there is substantial time and a high degree of proficiency required in properly setting the bag between the shell and the tank.